This is, of course, extraordinary — a minority government deliberately preventing M.P.s from scrutinizing one of the country’s biggest decisions in memory — and steers close to authoritarianism. There is no conceivable democratic mandate for crashing out of Europe without a deal. Indeed, various members of the 2016 campaign to leave the European Union frequently ruled out this possibility. Mr. Johnson himself said a few months ago that the odds of no deal were “a million to one against.” Forecasters predict crashing out will do the country great harm.

Yet this is the strange place in which Britain finds itself. Here’s the problem. As with “Mornington Crescent,” the rules on which its politics are based are unwritten — in contrast to its French, American or German counterparts — and its smooth workings are held together by convention, good manners and a sense of whether or not something is fair play. There are also few correctives, it is finding out, should someone decide to break with all this.